Determination of the refractive index of indigo dye thin film/flexible polyacetate substrate

A thin-film (215 nm) of indigo dye has been deposited on a polymer substrate (polyacetate) using thermal evaporation. The structure of the prepared film has been investigated using X-ray diffraction, showing X-ray amorphous or fine nanocrystalline nature. Analysis of the absorption coefficient in the strong absorption regions shows two indirect optical transitions with energy gaps (2.45 and 1.64 eV) in addition to the energy gap of the substrate (3.5 eV). Values of the refractive index have been calculated using four different methods. The average refractive index values near the first and second absorption edges are 2.525 and 2.871, respectively. The average values of the optical dielectric constant near the two corresponding edges are 6.377 and 8.064. Furthermore, the corresponding values of the nonlinear optical susceptibilities and nonlinear refractive index are in the same order of magnitude as other promising organic materials. This recommends indigo-thin films for flexible organic electro-optical applications.